Polymeric foams, in particular polyurethane foams are well-known and in general their preparation requires the mixing of liquid reactive chemical components, such as a polyole and an isocyanate, with additives such as a catalyst, a surfactant and a foaming agent for controlling the cellular structure of the foam, for example water which chemically reacts with the isocyanate in order to produce the required quantity of carbon dioxide necessary for the expansion of the foam. Moreover, to allow the foaming to start, it is necessary to perform the dispersion, in the liquid mixture, of a certain quantity of a gaseous nucleation agent, such as nitrogen, air or other inert gas which must be finely subdivided into "nucleides" or small-size bubbles suitable for promoting the foaming process.
In the continuous production of foams, in particular in the production of flexible foam slab-stocks as is currently performed by conventional methods, a thin layer of a polyurethane mixture in the liquid state is poured or spread on a moving substrate provided on a conveyor, allowing the foam to freely rise owing to the formation of gaseous CO.sub.2 caused by the chemical reaction between the chemical components, until total expansion of the foam is obtained. Conventional processes and apparatuses are described for example in the U.S. Pat. No. 3,325,823, U.S. Pat. No. 4,492,664 and U.S. Pat. No. 3,184,419.
The pre-expansion of the polyurethane mixture by means of a low-boiling blowing agent, which is preferably inert, for example liquid CO.sub.2 --more commonly known as "frothing"--is also a well-known technique and proposed in varying forms in polyurethane technology; this technology has been recently improved by the same Applicant with regard to the continuous production of polyurethane slab stocks in order to overcome numerous drawbacks which occur with apparatuses or with prior known processes which in practice have proved difficult to carry out.
In particular, according to the technology developed by the same Applicant, which has formed the subject of prior patent applications EP-A-0 645 226, it has been established that in the production of polyurethane foams the use of pressure controlled conditions during mixing positively influences the formation of the cells during the subsequent pre-expansion or frothing phase, before the chemical reaction between the components and polymerisation of the polyurethane mixture will start.
In the practical implementation of the foaming processes using the frothing technique by means of a low-boiling inert blowing agent, in general it has also been proposed to use mixers of mechanical type, in which mixing of the chemical components, blowing agent and nucleation gas occurs by a stirring action along a path which extends substantially in the axial direction inside the mixer, in the direction in which the same mixture flows towards a delivery opening. Mechanical mixers with a substantially axial progression of the flow during the mixing are described in numerous prior patents and in particular in U.S. Pat. No. 3,184,419.
In mechanical mixers of this type, however, there is a very rapid flowing of the mixture along the axial path in the mixing chamber, where the mixture is subjected to a stirring action conjointly to a strong centrifugal action tending to cause separation between the gaseous phase of the nucleation agent and the liquid phase of the polyurethane components; this results in the formation of preferential flows of separated components and irregular bubbles of nucleation gas, which give rise to the formation of large cavities or pin-holes in the polyurethane foam negatively influencing the quality and characteristics of the same foam. In practice it has been established that, there may be the formation of both small cavities or bubbles in a substantially homogeneous cellular structure, and large-size cells which undoubtedly originated inside the mixer.